In any case, you will want to apply the makeup to your appliance while it's on your face, rather than it off. The most important thing to remember in using makeup with foam latex/slushcast, is that it must contain castor oil. Anything else will destroy the appliance. Waterbased makeups can be used without problems as well. If you want to use something that doesn't contain castor oil, like mineral oil, pick up on some castor sealer and apply, or just put a thin layer of castor oil on the appliance.

With some larger appliances, you may want to apply the makeup with it off, so you can see some details better than in the mirror.

A most interesting little tidbit of info here… The womens makeup department of various stores will have some castor oil-based items, like eyebrow pencils. Don't laugh. There's some suiters out there who actually use this stuff every now and then. These are cheaper than the ones at the costume shops, and are great for detailing on the appliances.

Great-looking makeup takes time, and sometimes you will not want to use your fingers to do this. Latex triangles and brushes are great for the application, as well as the blending. When boogi applies the Mutant Cat makeup, it's not unusual for him to use several of these. It takes him around 60-90 minutes just to put the appliance on and do the makeup. Straight makeup can take at least 30-45 minutes, though it is not unusual for a good detailed job to take 1-2 hours. A good makeup job takes time.

In any case, you will want to 'set' the makeup, by applying powder, and brushing off the excess. This will keep the makeup from becoming all smeary and sticky to the touch. Use a powder puff and facial brush, which can most easily be found in the makeup area of most stores. Never mind that it's claimed to be for women, it's great for this.

Not sure who wrote this: The Bob Kelly Company makes a make-up called “Rubber Mask Greasepaint” that is designed to work on latex masks and appliances. It comes in a variety of colors, and most theatrical supply house carry it. Most acrylic make-ups (i.e. water-based) will work on latex, but not all of them will adhere well. Experiment first.